1. Field of the Invention
The present invention relates to a technique for broadcasting individual contract information of a digital content.
2. Description of the Related Arts
In the field of digital broadcasting, a pay broadcast system has recently been implemented in which only receiver under a contract with a broadcasting company is allowed to watch a digital content (hereinafter also referred to as a content). The pay broadcast system determines whether or not a receiver is a receiver under a contract by sending individual contract information to a receiver for which the contract has been signed and allows only receiver which has valid individual contract information to decode an encrypted content.
In the pay broadcast system, a method has been known in which a unique master key is assigned to each receiver, and individual contract information is sent encrypted with the master key corresponding to a receiver (see the non-patent document 1) as a method for sending individual contract information to a receiver. The individual contract information is sent as an EMM (Entitlement Management Message), and an expire date of about one month is set for the individual contract information. The individual contract information is decoded with the master key in the receiver which has received the individual contract information, whereby decoding of the content is allowed. Further, if a subscriber continues the contract, new individual contract information is transmitted to the receiver and the contract is updated in the receiver.
As another method, a method has been also known in which a work key which is common to receivers is provided to the receivers in advance, and individual contract information to which a receiver ID indicating a destination receiver is added is sent encrypted with the work key as an ECM (Entitlement Control Message), which is common information (see the patent document 1). The receiver decodes the encrypted individual contract information with the work key, and if the receiver ID added to the individual contract information coincides with the receiver ID of the receiver, the receiver obtains the individual contract information. If the receiver ID added to the individual contract information is the receiver ID of another receiver, the receiver does not obtain the individual contract information. The individual contract information is transmitted in response to the contract status, for example, at the time of contract or cancellation of contract. In order to prevent falsified individual contract information from being used by a genuine receiver, the individual contract information has a digital signature. As the digital signature is verified in the receiver, the falsified individual contract information can be excluded.    Patent document 1: Japanese Patent Number 3561154 (paragraphs 0010 to 0037)    Non-patent document 1: Association of Radio Industries and Businesses “Standard Access Control Method in a Digital Broadcasting” ARIB STD-B25, Part 1